Ceramic tools have recently been used for cutting high hardness materials such as roll materials. In this cutting processing, however, since the hardness of the work piece is very high, the cutting edge becomes hot. With the conventional high purity alumina tools, therefore, plastic deformation and chipping abrasion of the cutting edge occur because their thermal conductivity is small.
The specific example of the roll material involves Nodular cast iron and the composition thereof contains 3.0 to 3.3% by weight of C, 0.68% by weight of Cr, 3.5% by weight of Ni, 0.65% by weight of Mo and the balance Fe.
It is known, as described in Japanese Patent Application (OPI) No. 89410/75 (the term "OPI" is used herein to mean a "published unexamined Japanese patent application"), that Al.sub.2 O.sub.3 -TiN-based ceramic tools prepared by sintering an Al.sub.2 O.sub.3 /TiN mixture (with a TiN content of from 5 to 40% by volume) by a hot pressing method or a hot isostatic pressing method (HIP process) have very high antioxidation and anticrator wear properties and are excellent as cutting tools. However, since the hot pressing method involves a combination of steps of molding and sintering a starting mixture in a graphite mold and of cutting and working the thus obtained ceramic plate with a diamond whetstone, it suffers from the disadvantage that when used for commercial production, it increases production costs, and therefore the hot pressing method is not suitable for mass production. The HIP process generally involves a step pressing a molded article to be sintered or a sintered article with pressurized gas to prepare a dense product. When the molded article or a sintered article is porous because the pressurized gas permeates inside the article, the article cannot be densified at all. Therefore, it is necessary to apply the HIP process after covering the surface of the article or previously sintering the article to density of at least 95% wherein closed air pores are formed. This makes difficult the commercial production of ceramic tools by the HIP process.
Japanese Patent Application (OPI) No. 126566/80 discloses a process for the production of ceramic materials for cutting tools, containing from 80 to 95% Al.sub.2 O.sub.3, with the balance being TiN, ZrC, Zr, and/or ZrN. These ceramics, however, are also not satisfactory in performance; for example, thermal conductivity thereof is small because of the small amount of TiN added to increase the thermal conductivity, and the strength is insufficient when the ceramic materials are used for cutting of high hardness materials such as roll materials, even though the plastic deformation and chipping abrasion properties of the cutting edge is considerably increased over conventional high purity alumina tools.